Variable-crown rolls are needed and used in paper machines, for example, in presses and in calenders, and, further, such rolls are used in paper finishing devices, such as supercalenders. Thus, it is almost an essential feature of such variable-crown rolls that the roll forms a nip with a backup roll whereby a paper web is passed through the nip, e.g., to be dewatered. The roll is provided with necessary crown variation means, by whose means the roll mantle is loaded in the direction of the nip plane and by whose means the nip profile is controlled.
In the past, it was the most common construction in variable-crown rolls to mount the tubular roll mantle of the roll from its ends on the roll axle by means of roller bearings. Such a conventional mode of journalling also had its advantages, for example, the journalling can be accomplished in a rather simple manner and so far the cost of this construction has been considered to be relatively reasonable. Such a conventional mode of journalling, in which the roll mantle is mounted from its ends stationarily on the axle, is however not totally suitable for even nearly all applications in paper machines. From variable-crown rolls which are in nip contact with a backup roll, quite often such a property is required that the roll mantle must be able to move in the radial direction in relation to the roll axle. In view of regulating the profile of linear load across the entire axial length of the roll, the roll ends must also be able to move in the radial direction in relation to the axle. For this reason, rolls have been developed in which this property has been accomplished so that the end bearings of the roll mantle have been mounted on separate annular parts, which can move radially in relation to the roll axle. One such roll is described, for example, in European Patent No. 0,332,594.
Mounting of a roll by means of roller bearings has, however, caused a number of drawbacks and problems for the manufacture and operation of the roll. These drawbacks include the numerous machinings required by the roller bearings, the problems arising from wear of the bearings, limitations imposed by the roller bearings in respect of the oil used in the roll, limitations on the web running speeds with roller bearings, and accuracy of rolling of the bearings. For these reasons, there has been a desire to abandon the roller bearings, and in recent years variable-crown rolls have been developed in which the journalling of the roll mantle has been accomplished by means of glide bearings. Such rolls with glide bearings have been described, for example, in the U.S. Pat. Nos. 5,060,357 and 5,111,563 and in Finnish Patent Application Nos. 941107 (which corresponds to U.S. Pat. No. 5,509,883, the entire disclosure of which is incorporated by reference herein), 941991, 944272 and 950814 (which corresponds to U.S. patent application Ser. No. 08/411,439, the entire disclosure of which is incorporated by reference herein). It is however a problem in rolls with glide bearings that the end areas of the roll may be heated excessively, which has a highly detrimental effect on the nip profile. Heating of the end areas is, of course, a problem also in variable-crown rolls of other types which form a nip with a backup roll through which a fibrous web runs. One important factor in the excessive heating of the end areas is, among other things, the fact that in the middle area of the roll the heat can be conducted away from the roll through the paper web which runs over the middle area of the roll, whereas the ends are heated, because the web is often substantially narrower than the roll mantle. Thus, there is no medium such as a moving web to convey heat from the ends of the roll.
In a roll with glide bearings, the risk of heating of the end areas can be even higher because the generation of heat in a glide bearing is higher than in a roller bearing. The generation of heat in the ends in a roll with glide bearings is a particular problem also because several glide bearing elements are effective at the same location on the inner circumference of the roll mantle. Part of the heat that is generated is, of course, carried away from the areas of the ends along with the oil flowing through the glide bearing, but in normal cases the quantity of oil is not sufficiently large to carry away the required quantity of heat.